Out-of-ground swimming pools are popular because they are much less expensive than in-ground pools. Indeed, it is not necessary to dig any holes or alter the ground in any way, if there is a flat area large enough to receive the out-of-ground pool.
A problem with out-of-ground pools is that they do not take advantage of the beneficial insulating soil around them as do in-ground pools, since the pool walls are surrounded by the atmospheric air. The walls of out-of-ground pools are often made of a metallic sheet, e.g. aluminium, and consequently do not have good insulating properties to keep the water warm. Indeed, the pool water will often become much colder by loosing its heat through the pool walls, the metal (such as aluminium) being a good thermal conductor.
A solar sheet is often used as a heat insulating barrier and to prevent water evaporation which has a cooling effect on the water. It also heats the pool water through the instrumentality of the sun rays, the sheet floating on the water surface and concentrating the sun rays in the water. However, despite the use of a sheet, a considerable amount of heat loss takes place through the pool walls. It consequently costs more to the pool owner for heating his pool water.